1. Field of the Invention
The invention relates to metal oxide powders with a bimodal particle size distribution or to bimodal ceramic-binder material composites, to ceramics that can be made from these metal oxide powders or composites, especially milling ceramics for use in dental technology, to methods for producing of the metal oxide powders and of the ceramics, to the use of nanoscale metal oxide powders for producing the metal oxide powders and of the ceramics, and to dental ceramic products.
2. Related Technology
Ceramics made of metal oxide powders, especially Al2O3, have been in use for some time in dental technology because of their stability under load and their biocompatibility. Partially stabilized ZrO2 has also been considered since, due to its polymorphic state, it has greater mechanical strength than Al2O3. These ceramics are processed by means of a milling cutter, whereby either a green compact, a pre-sintered body, a final-sintered porous body (with subsequent glass infiltration), or a final-sintered solid ceramic body is subject to machining. To start with, the metal oxide powders are compacted under pressure. For this purpose, cold isostatic or uniaxial pressing methods are commonly employed, whereby, due to the inevitable density gradient in comparison to the CIP (cold isostatic pressing) process, uniaxial pressing does not allow a uniform density.
As an alternative to this production of green compacts, companies in the dental industry, for example, Metoxit, supply ceramic blocks treated by a hot isostatic process. Here, the ceramic starting powder is simultaneously compacted and sintered. This results in the highest compacting which, at 6.065 g/cm3, comes close to the theoretical density in the case of, for example, ZrO2 doped with 3 mole-% of Y2O3. However, this method is very costly and it yields ceramic blocks that, due to their high density, can take up to six hours to be made into a finished three-part dental bridge in a milling process, for instance, using a dental milling cutter made by the DCS Company.
Dentsply Degussa Dental offers an alternative method. Here, the compacted green compact is first machined, taking into account a margin for shrinkage, and it subsequently undergoes final sintering. However, milling the green compact encounters problems because of the relatively low green density of monomodal powders since fractures can often occur during machining. Shipping the green compacts to dental laboratories that will process them is problematic because of their non-optimal green density. Furthermore, due to the great shrinkage, it is also problematic to set a deviation from the isotropic shrinkage that is still acceptable for dental requirements. Furthermore, the high sintering temperature and the long sintering duration have proven to be disadvantageous for practical use since, for example, these factors lead to greater stress and greater thermal wear of the heating elements, or else expensive types of furnaces must be used.
The low shrinkage of the bimodal metal oxide powder according to the invention also allows a better setting of an approximately isotropic shrinkage, especially of free-form surfaces. Moreover, the greater packing density accounts for lower shrinkage, as a result of which the green compacts take up less space during transportation as well as in the sinter furnace. If need be, the sintering temperature can also be lowered, without detrimentally diminishing the strength of the product to a level below that required for dental applications.
The In-Ceram method of the Vita Company includes the production of final-sintered porous ceramic blocks that can also be machined using low-power dental milling cutters. In order to attain the strength needed for use, the porous body is infiltrated with lanthanum glass, whereby the infiltration temperature lies below the sintering temperature of the porous final-sinter ceramic body and, consequently, shrinkage is almost completely avoided. The problems encountered here are the quite low strength of the porous final-sinter ceramic body (limited handling) as well as the non-optimal strength of the ceramic-glass composite after the infiltration. Doping with nanoscale ceramic powder brings about an increase in strength of the porous final-sinter ceramic block that functions as the skeleton.
In order to lower the sintering temperature during the production of dental milling ceramics, it has been proposed to use so-called nanoscale metal oxide powders, that is to say, metal oxide powders, whose average particle sizes lie in the nanometer range instead of the usual metal oxide powders whose particle sizes are greater than 1 μm. However, the handling and processing of these “nano-powders” have proven to be difficult in actual practice. Thus, their high sintering activity can cause undesired agglomeration and increased grain growth. Moreover, the low bulk density or tap density often renders the shaping procedure difficult. Consequently, there is so much technical effort involved in creating a nanoscale structure that satisfactory profit margins cannot be attained on the dental market. Furthermore, the use of pure nanoscale metal oxide powders is not feasible due to their high production costs. A special effect of a nanoscale structure, however, has proven to be very advantageous for the dental industry. If the particle boundary range of the structure of the sintered sample is below 1/20 of the wavelength of visible light, then it will be transparent. In actual practice, among other things, a particle size that lies below the wavelength of visible light leads to a more or less pronounced translucence. This translucence is also improved by especially chemically pure starting materials, since, for example, no impurities can become deposited on the skeleton ceramic. In dental practice, greater translucence of the skeleton ceramic means that a thinner ceramic layer is needed which, on the one hand, makes it easier to achieve optimal esthetics and, on the other hand, brings about less abrasion of the natural teeth that serve to anchor a dental bridge.